9. Electron Beam Welding

9. Electron Beam Welding


120 The application of highly accelerated elechigh voltage supply

beam generation beam forming and guidance

fusion, drilling and welding process and also

cathode control elektrode

for surface treatment has been known since

anode

the Fifties. Ever since, the electron beam

adjustment coil to vacuum pump

valve

trons as a tool for material processing in the

welding process has been developed from the laboratory stage for particular applications. In

viewing optics

stigmator

focussing coil

this cases, this materials could not have been

defelction coil

joined

by any industrially applied

high-

working chamber

production joining method. The electron beam welding machine is made

workpiece workpiece handling

to vacuum pump

beam generation, beam manipulation and forming and working chamber. These compo-

chamber door br-er9-01e.cdr

up of three main components:

© ISF 2002

Schematic Representation of an Electron Beam Welding Machine

nents may also have separate vacuum systems, Figure 9.1.

Figure 9.1 A tungsten cathode which has been heated under vacuum emits electrons by thermal emission. The heating of the tungsten cathode may be carried out directly - by filament cur-

power supply

chamber evacuation system valve

evacuation system for gun

control cabinet

EB-gun

rent - or indirectly - as, for example, by coiled filaments. The electrons are accelerated by high voltage between the cathode and the pierced anode. A modulating electrode, the socalled “Wehnelt cylinder”, which is positioned between anode and cathode, regulates the electron flow. Dependent on the height of the cut-off voltage between the cathode and the

working chamber workpiece receiving platform workpiece handling

modulating electrode, is a barrier field which may pass only a certain quantity of electrons.

control panel control desk

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© ISF 2002

All-Purpose EBW Machine and Equipment

This happens during an electron excess in front of the cathode where it culminates in Figure 9.2

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121

form of an electron cloud. Due to its particular shape which can be compared to a concave mirror as used in light optic, the Wehnelt cylinder also effects, besides the beam current adjustment, the electrostatic focussing of the electron beam. The electron beam which diverges after having passed the pierced anode, however, obtains the power density which is necessary for welding only after having passed the adjacent alignment and focussing system. One or several electromagnetic focussing lenses bundle the beam onto the workpiece inside the vacuum chamber. A deflection coil assists in maintaining the electron beam oscillating motion. An additional stigmator coil may help to correct aberrations of the lenses. A viewing optic or a video system allows the exact positioning of the electron beam onto the weld groove. The core piece of the electron beam welding machine is the electron beam gun where the electron beam is generated under high vacuum. The tightly focussed electron beam diverges rapidly under atmospheric pressure caused by scattering and ionisation development with air. As it would, here, loose power density and efficiency, the welding process is, as a rule, carried out under medium or high vacuum. The necessary vacuum is generated in separate vacuum pumps for working chamber and beam gun. A shut-off valve which is positioned between electron gun and working chamber serves to maintain the gun vacuum while the working chamber is flooded. In universal machines, Figure 9.2, the workpiece manipulator assembly inside the vacuum chamber is a slide with working table positioned over NCcontrolled stepper motors. For workpiece removal, the slide is moved from the vacuum chamber onto the workpiece platform. A distinction is made between electron beam machines with vertical and horizontal beam manipulation systems.

back-scattered electrons

The energy conversion in

x-ray

the workpiece, which is thermal radiation secondary electrons

schematically

shown

in

Figure 9.3, indicates that the kinetic energy of the

x

convection

highly

accelerated

elec-

trons is, at the operational y

point, not only converted

heat conduction

into the heat necessary for

z © ISF 2002

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Energy Transformation Inside Workpiece

welding, but is also released by heat radiation

Figure 9.3 2005


122

and heat dissipation. Furthermore, a part of the incident electrons (primary electrons) is subject to backscatter and by secondary processes the secondary electrons are emitted from the workpiece thus generating X-rays. The impact of the electrons, which are tightly focussed into a corpuscular beam, onto the workpiece surface stops the electrons; their penetration depth into the workpiece is very low, just a few µm. Most of the kinetic energy is released in the form of heat. The high energy density at the impact point causes the metal to evaporate thus allowing the following electrons a deeper penetration. This finally leads to a metal vapour cavity which is surrounded by a shell of fluid metal, covering the entire weld depth, Figure 9.4. This deep-weld nowadays a)

b)

c)

d)

effect

allows

penetration

depths into steel materials of up to 300 mm, when

© ISF 2002

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modern high vacuum-high Principle of Deep Penetration Welding

voltage machines are used.

Figure 9.4 The diameter of the cavity corresponds approximately electron beam

motion of the molten metal groove melting pool welding direction

with the beam diameter. By groove front side

a relative motion in the di-

keyhole molten zone

vapour capillary

rection of the weld groove between F1

solidified zone

F1 : force resulting from vapour pressure F2 : force resulting from surface tension F3 : force resulting from hydrostatic pressure

workpiece

and

electron beam the cavity

F2

penetrates through the ma-

F3

terial, Figure 9.5. At the

F1

front side of the cavity new material is molten which, to © ISF 2002

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Condition in Capillary

some extent, evaporates, but for the most part flows

Figure 9.5 2005


123

around the cavity and rapidly solidifies at the backside. In order to maintain the welding cavity open, the vapour pressure must press the molten metal round the vapour column against the cavity walls, by counteracting its hydrostatic pressure and the surface tension. However, this equilibrium of forces is unstable. The transient pressure and temperature conditions inside the cavity as well as their respective, momentary diame-

β

ters are subject to dynamic

I

II

III

changes. Under the influence of the resulting, dy-

workpiece movement

namically changing geome© ISF 2002

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try of the vapour cavity and

Model of Shrinkage Cavity Formation

with an unfavourable selecFigure 9.6 tion of the welding parameters, metal fume bubbles may be included which on cooling turn into shrinkholes, Figure 9.6.

150

The unstable pressure exposes the molten backside of the vapour cavity to strong and irregular changes in shape (case II). Pressure EBW MSG UP (narrow gap)(narrow gap) EBW

variations interfere with the regular flow at the

UP (conventional)

MSG (narrow gap)

UP (narrow gap)

UP (conventional)

welding current

0,27 A

260 A

650 A

510 A

welding voltage groove area

150.000 V

30 V

30 V

28 V

cavity backside, act upon the molten metal and, in the most unfavourable case, press the

number of passes

896 mm 1

2098 mm 35

4905 mm 81

5966 mm 143

unevenly distributed molten metal into differ-

filler metal

0

23 kg

54 kg

66 kg

melting efficiency

7,7 kg/h

ent zones of the molten cavity backside, thus

energy input

64·10 kJ

128·10 kJ

293·10 kJ

377·10 kJ

welding time

27 min

4 h 35 min

4 h 11 min

7 h 20 min

2

3

2

5 kg/h

2

13 kg/h 3

3

2

9 kg/h 3

forming the so-called vapour pockets. The cavities are not always filling with molten

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© ISF 2002

Comparison of EB, GMAW and SAWNarrow Gap and Conventional SAW

metal, they collapse sporadically and remain as hollow spaces after solidification (case Ill). The angle ß (case I) increases with the rising

Figure 9.7 2005


124

weld speed and this is defined as a turbulent process. Flaws such as a constantly open vapour cavity and subsequent continuous weld solidification could be avoided by selection of job-suitable welding parameter combination and in particular of beam oscillation characteristics, it has to be seen to a constantly of the molten metal, in order to avoid the abovementioned defects. Customary beam oscillation types are: circular, sine, double parabola or triangular functions. Thick plate welding accentuates the process-specific advantage of the deep-weld effect and, with that, the possibility to join in a single working cycle with high weld speed and low heat input quantity. A comparison with the submerged-arc and the gas metal-arc welding processes illustrates the depth-to-width ratio which is obtainable with the electron beam technology, Figure 9.7. Electron beam welding of thick plates offers thereby decisive advantages. With modern equipment, wall thicknesses of up to 300 mm with length-to-width ratios of up to 50 : 1 and consisting of low and high-alloy materials can be welded fast and precisely in one pass and without adding any filler metal. A corresponding quantification shows the advantage in regard of the applied filler metal and of the primary energy demand. Compared with the gas-shielded narrow gap welding process, the production time can be

in vacuum

reduced by the factor of approx. 20 to 50.

thin and thick plate welding (0,1 mm bis 300 mm)

extremely narrow seams (t:b = 50:1)

low overall heat input => low distortion => welding of completely processed components

high welding speed possible

no shielding gas required

practice, Figure 9.8. Pointing to series produc-

high process and plant efficiency

tion, the high profitability of this process is

material dependence, often the only welding method

dominant. This process depends on highly

Numerous specific advantages speak in favour of the increased application of this high productivity process in the manufacturing

energetic efficiency together with a sparing

at atmosphere

very high welding velocity

good gap bridging

no problems with reflection during energy entry into workpiece

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use of resources during fabrication.

© ISF 2002

Advantages of EBW

Figure 9.8 2005


125 Considering the

above-mentioned

advan-

tages, there are also disadvantages which in vacuum

emerge from the process. These are, in par-

electrical conductivity of materials is required

high cooling rates => hardening => cracks

ticular, the high cooling rate, the high equip-

high precision of seam preparation

ment costs and the size of the chamber, Fig-

beam may be deflected by magnetism

ure 9.9.

X-ray formation

size of workpiece limited by chamber size

high investment

In accordance with DIN 32511 (terms for methods and equipment applied in electron

at atmosphere

and laser beam welding), the specific desig-

X-ray formation

limited sheet thickness (max. 10 mm)

high investment

nations, shown in Figure 9.10, have been standardised for electron beam welding.

small working distance

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© ISF 2002

Disadvantages of EBW

by their working vacuum quality or the unit concept but also by the acceleration voltage level, Figure 9.11. The latter exerts a consid-

Figure 9.9

erable influence onto the obtainable welding

results. With the increasing acceleration voltage, the achievable weld depth and the depth-towidth ratio of the weld geometry are also increasing. A disadvantage of the increasing accelerating voltage is, however, the exponential increase of X-rays and, also, the likewise increased sensitivity to flash-over voltages. In correspondence with the size of the workpiece to be welded and the size of groove

the chamber volume, high-

in

industrial

production,

while the low-voltage technology (max. 60 kV) is a good alternative for smaller units and weld thicknesses. The design of the unit for the low-voltage

technique

gt

a se

m

width of seam

g len

t

fs ho

m ea

weld thickness

of up to 200 kW are applied

len

f ho

weld penetration depth

(150 - 200 kV) with powers

Nahtdicke

generators

weld reinforcement

beam

root reinforcement

voltage

end crater

upper bead

blind bead molten area

unapproachable gap lower bead

root weld © ISF 2002

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Basic Definitions

is Figure 9.10

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126

simpler as, due to the lower acceleration voltage, a separate complete lead covering of the unit is not necessary.

by accelerating voltage: high voltage machine (UB=150 kV) low voltage machine (UB=60 kV)

-6

< 1 x 10 mbar

< 5 x 10-4 mbar

by pressure: high vacuum machine fine vacuum machine atmospheric machine (NV-EB welding)

by machine concept: conveyor machine clock system all-purpose EBW machine local vacuum machine mobile vacuum machine micro and fine welding machine br-er9-20e.cdr

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Classification of EBW Machines

Figure 9.11

EB-Welding in High Vacuum

Figure 9.12

While during the beam generation, the vacuum (p = 10-5 mbar) for the insulation of the beam generation compartment and the prevention of cathode oxidation is imperative, the possible working pressures inside the vacuum chamber vary between a high vacuum (p = 10-4 mbar) and atmospheric pressure. A collision of the electrodes with the residual gas molecules and the scattering of the electron beam which is connected to this is, naturally, lowest in high vacuum. The beam diameter is minimal in high vacuum and the beam power density is maximum in high vacuum, Figure 9.12. The reasons for the application of a high vacuum unit are, among others, special demands on the weld (narrow, deep welds with a minimum energy input) or the choice of the materials to be welded (materials with a high oxygen affinity). The application of the electron beam welding process also entails advantages as far as the structural design of the components is concerned.

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With a low risk of oxidation and reduced demands on the welds, the so-called “mediumvacuum units” (p = 10-2 mbar) are applied. This is mainly because of economic considerations, as, for instance, the reduction of cycle times, Figure 9.13. Areas of application are in the automotive industry (pistons, valves, torque converters, gear parts) and also in the metalworking industry (fittings, gauge heads, accumulators). Under extreme demands on the welding time, reduced requirements to the weld geometry, distortion and in case of full material compatibility with air or shielding gas, out-of-vacuum welding units are applied, Figure 9.14. Their advantages are the continuous welding time and/or short cycle times. Areas of application are in the metal-working industry (precision tubes, bimetal strips) and in the automotive industry (converters, pinion cages, socket joints and module holders).

< 1 x 10-6 mbar

-4

< 1x 10 mbar

< 5 x 10-2 mbar ~ 10

-1

mbar

~ 1 mbar

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EB-Welding in Fine Vacuum

Figure 9.13

Atmospheric Welding (NV-EBW)

Figure 9.14

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128

A further distinction criterion is the adjustment of the vacuum chambers to the different joining tasks. Universal machines are characterised by their simply designed working chamber, Figure 9.15. They are equipped with vertically or horizontally positioned and, in most cases, travelling beam generators. Here, several workpieces can be welded in subsequence during an evacuation cycle. The largest, presently existing working chamber has a volume of 265 m³.

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EBW Clock System Machine

Machine Concept - Conventional Plant

Figure 9.15

Figure 9.16

Clock system machines, in contrast, are equipped with several small vacuum chambers which are adapted to the workpiece shape and they are, therefore, characterised by short evacuation times, Figure 9.16. Just immediately before the welding starts, is the beam gun coupled to the vacuum chamber which has been evacuated during the preceding evacuation cycle, while, at the same time, the next vacuum chamber may be flooded and charged/loaded.

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Conveyor machines allow the continuous production of welded joints, as, for example, bimetal semi finished products such as saw blades or thermostatic bimetals, Figure 9.17. In the main chamber of these units is a gradually raising pressure system as partial vacuum pre and post activated, to serve as a vacuum lock.

butt weld

T-joint/ fillet weld

a)

T-joint butt welded

b)

lap weld

semi-finished material

endproduct

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Seam Appearance for EB-Welding in Vacuum

EBW Conveyor Machine

Figure 9.17

© ISF 2002

Figure 9.18

Systems which are operating with a mobile and local vacuum are characterised by shorter evacuation times with a simultaneous maintenance of the vacuum by decreasing the pumping volume. In the “local vacuum systems”, with the use of suitable sealing, is the vacuum produced only in the welding area. In “mobile vacuum systems” welding is carried out in a small vacuum chamber which is restricted to the welding area but is travelling along the welded seam. In this case, a sufficient sealing between workpiece and vacuum chamber is more difficult. With these types of machine design, electron beam welding may be carried out with components which, due to their sizes, can not be loaded into a stationary vacuum chamber (e.g. vessel skins, components for particle accelerators and nuclear fusion plants). 2005


130

In general the workpiece is moved during electron beam welding, while the beam remains stationary and is directed onto the workpiece in the horizontal or the vertical position. Depending on the control systems of the working table and similar to conventional welding are different welding positions possible. The weld type preferred in electron beam welding is the plain butt weld. Frequently, also centring allowance for centralising tasks and machining is made. For the execution of axial welds, slightly oversized parts (press fit) should be selected during weld preparation, as a transverse shrinkage sets in at the beginning of the weld and may lead to a considerable increase of the gap width in the opposite groove area. In some cases also T-welds may be carried out; the T-joint with a plain butt weld should, however, be chosen only when the demands on the strength of the joints are low, Figure 9.18. As the beam spread is large under atmosphere, odd seam formations have to be considered during Non-Vacuum Electron Beam Welding, Figure 9.19. In order to receive uniform and reproducible results with electron beam welding, an exact knowledge about the beam geometry is necessary and a prerequisite for: - tests on the interactions between beam and substance - applicability of welding parameters to br-er9-18e.cdr

© ISF 2002

Seam Appearence at Atmospheric Welding (NV-EBW)

other welding machines - development of beam generation systems.

Figure 9.19 The objective of many tests is therefore the exact measurement of the beam and the investigation of the effects of different beam geometries on the welding result. For the exact measurement of the electron beam, a microprocessor-controlled measuring system has been developed in the ISF. The electron beam is linearly scanned at a high speed by means of a point probe, which, with a diameter of 20 µm is much smaller than the beam diameter in the focus, Figure 9.20. When the electron beam is deflected through the aperture diaphragm located inside the sensor, the electrons flowing through the diaphragm

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are picked up by a Faraday shield and diverted over a precision resistor. The time progression of the signal, intercepted at the resistor, corresponds with the intensity distribution of the electron beam in the scanning path. In order to receive an overall picture of the power density distribution inside the electron beam, the beam is line scanned over the slit sensor (60 lines). An evaluation program creates a perspective view of the power density distribution in the beam and also a two-dimensional representation of lines with the same power density.

hole sensor hole with aperture diaphragm Faraday cup (20 µm)

track of the beam

cross section of the beam

measurement field

slit sensor

slit with Faraday cup

FILENAME: R I N G S T R Accel. voltage: 150 kV Beam current: 600 mA Prefocus current: 700 mA Main focus current: 1500 mA Cath. heat current: 500 mm Max. Density: 26,456 kW/mm2 2 Ref. Density: 26,456 kW/mm

voltage

cross section of the beam

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Two Principles of Electron Beam Measuring

Figure 9.20

© ISF 2002

Energy Concentration and Development in Electron Beam

Figure 9.21

An example for a measured electron beam is shown in Figure 9.21. It can be seen clearly that the cathode had not been heated up sufficiently. Therefore, the electrons are sucked off directly from the cathode surface during saturation and unsaturated beams, which may lead to impaired welding results, develop. During the space charge mode of a generator, the electron cloud is sufficiently large, i.e., there are always enough electrons which can be sucked off. In the ideal case, the developed power density is rotationally symmetrical and in accordance with the Gaussian distribution curve. The electron signals are used for the automatic seam tracking. These may be either primary or secondary electrons or passing-through current or the developing X-rays. When backscattered primary electrons are used, the electron beam is scanned transversely to the groove. A 2005


132

computer may determine the position of the groove relative to the beam by the signals from the reflected electrons. In correspondence with the deflection the beam is guided by electromagnetic deflection coils or by moving the working table. This kind of seam tracking system may be used either on-line or off-line. The broad variation range of the weldable maindustrial areas: automotive industries aircraft and space industries mechanical engineering tool construction nuclear power industries power plants fine mechanics and electrical industries job shop material: almost all steels aluminium and its alloys magnesium alloys copper and its alloys titanium tungsten gold material combinations (e.g. Cu-steel, bronze-steel) ceramics (electrically conductive)

terials and also material thicknesses offer this joining method a large range of application, Figure 9.22. Besides the fine and micro welding carried out by the electronics industry where in particular the low heat input and the precisely programmable control is of importance, electron beam welding is also particularly suited for the joining of large crosssections.

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EBW Fields of Application

Figure 9.22

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